1. Field of the Invention
The present invention relates to a method for predicting an output current of a transistor, and more specifically, to a method for predicting an output current of a transistor when a direct current amplification factor of the transistor is varied depending on the output current of the transistor.
2. Description of the Prior Art
Parameters such as the direct current amplification factor vary among transistors due to variations in conditions in the manufacture process. In the product inspection performed after manufacturing, the manufacturers carry out a measurement of a direct current amplification factor H.sub.FE0 obtained at a rated (predetermined) voltage and current (e.g. 5 V, 5 mA) which is a condition of use and the transistors are delivered to the users together with the data.
For the users of the transistors who design driver circuits therefor, it is important to know how much output current is obtained when a parameter such as an input voltage is set in designing the circuits. It is therefore typical for the users to predict the output current in consideration of various conditions prior to designing.
An output current I.sub.D of a transistor part incorporating a resistor as shown in FIG. 1 is generally given by the following expression (1): ##EQU1## where V.sub.I is an input voltage to the transistor, V.sub.BE is the base-emitter voltage of the transistor, R.sub.1 is a resistor series connected to the input voltage to the transistor, R.sub.2 is a resistor parallel connected between the base and emitter of the transistor, H.sub.FE (I.sub.D) is a direct current amplification factor at the output current I.sub.D of the transistor, K.sub.2 is the temperature coefficient of the direct current amplification factor of the transistor, K.sub.3 is the temperature coefficient of the resistor R.sub.1, K.sub.4 is the temperature coefficient of the resistor R.sub.2, and K.sub.5 is the base-emitter temperature coefficient of the transistor.
While the output current of the transistor part incorporating a resistor is obtained by the expression (1) by setting the parameters such as the input voltage, the direct current amplification factor H.sub.FE (I.sub.D) varies depending on the value of the collector current (output current). Specifically, as shown in FIG. 2, in a transistor 1, it is typical that the direct current amplification factor H.sub.FE plunges from a point as the collector current I.sub.C increases and that when the collector-emitter voltage V.sub.CE decreases as the transistor 1 conducts, the direct current amplification factor H.sub.FE further decreases for the same collector current. In FIG. 2, G is an example where V.sub.CE is 5 V and H is an example where V.sub.CE is 0.3 V. For this reason, the value (point P of FIG. 2) required by the specification conditions cannot be used as the direct current amplification factor. Therefore, a current amplification factor H.sub.FE at an output current (collector current) predicted from the graph of FIG. 2 is obtained and based on the current amplification factor H.sub.FE, the output current I.sub.D is predicted by a calculation. However, when the calculated output current is different from the predicted output current, since they are based on different current amplification factors H.sub.FE, the predicted output current is re-set and the calculation is performed again. These must be repeated some times to correctly predict the output current I.sub.D. K.sub.3 and K.sub.4 are temperature coefficients which become constant and do not vary when the operating temperature is determined. K.sub.2 and K.sub.5 slightly vary according to the conditions of use but they can be considered constant with few errors since their variation is small.
As described above, according to the conventional output current predicting method, the complicated calculation is repeated many times to obtain an approximate value, so that the process takes a long time and is cumbersome and the output current cannot be accurately predicted.